Introduction

The purpose of the fourth gate was gain a better understanding for how the device used its parts to generate motion and to create design changes for the circular saw on a system level. To accomplish this, a complete reassembly of our product was done and each step was recorded in detail to be used in later analysis. This was crucial because it helped to gain an understanding in how each of the parts were assembled in accordance to the kind of motion they would be responsible for producing. In addition, this was also important when considering design revisions because they would ultimately impact the assembly process of the product.

Project Management: Coordination Review

\'\'\'Cause for Corrective Action\'\'\'

Overall, the points system we developed during gate 1 has served us very well and has been very useful in assigning group members a balanced work load. We used this again to assign parts of gate 4 for each group member to accomplish by a predetermined due date. This was necessary because the person compiling everything into the wiki was busy studying for multiple quizzes in other classes and required ample time to complete their task. While there have been a few complications due to the fall recess, we had still managed to successfully complete every part of gate 4. At this point, we are ready to move on to the gate revisions and finish the project as a whole. Our group had to think carefully and plan ahead since everyone would be separated during the fall recess. Some of the things that we did to help make the process run more efficiently were the following:

Emphasis on communication over the fall recess in order to discuss parts of gate 4 that members were having difficulty completing.

Creation of an outline of reassembly steps based on the product dissection done in gate 2.

Knowing what tools would be needed for the reassembly based on the dissection done in gate 2.

Creation of properly organized roles during the reassembly process.

Emphasis on proceeding with caution so that the product was not damaged, which would greatly impede the reassembly process.

Emphasis on working quickly due to time constraints on members during reassembly date.

The reassembly was successful, however there were some challenges that were faced along the way as well. The largest problem was completing everything while separated over the fall recess, but this was overcome by keeping in contact with each other and finishing parts little by little until they were completed by the time everyone had returned. Another challenge was that some group members couldn\'t make it to the initial date decided for the reassembly, so it was pushed back by two days. This made completing the gate somewhat more difficult because it narrowed the time window between the due date of the gate and the point when some members were finally able to complete parts of gate 4 with information attained from completing the reassembly.

The last step in the project is to make revisions to all of the gates and present our design revisions during the final week of class. This will be difficult because there are a lot of changes that need to be made on certain gates, but we plan to overcome this by splitting up the revisions among group members. For example, some members will be responsible for creating new diagrams, others will revise tables, and someone can come up with a way to improve the architecture of the wiki pages. In addition, finals week is approaching and will require us to work quickly and efficiently in order to complete this project and still have ample time to study.

Product Archaeology: Product Explanation

In order to assemble the product, the following tools are needed;

T20 Torx Screwdriver

13 mm Torque Wrench

Hammer

Hands

\'\'\'Product Assembly Guide\'\'\'

Ease of Reassembly

Difficulty Rating Scale

The difficulty scale for the product assembly is based on three different ratings. Each rating is based on a numerical scale ranging from one to three. A rating of one would be described as easy, a rating of two would be described as medium, and a rating of three would be described as hard. Each rating takes into consideration the complexity and amount of knowledge required in order to perform each step. Simpler tasks require less effort or technical knowledge while higher rated tasks may require advanced knowledge of the product or special tools at the user\'s disposal.

Difficulty Rating

Description

1

Requires little to no effort or knowledge prior to assembly (ex. removing screws or tightening something with a wrench). Step is performed in a relatively quick amount of time. Very low complexity.

2

Requires greater force to perform assembly step. May require stabilization of product (ex. multiple hands holding product or vice grip) or knowledge of product prior to assembly. Step may take some time to accomplish, but does not cause a serious interruption in the assembly process. Intermediate level of complexity.

3

Requires extreme force or tool not necessarily available to average person to perform assembly step (ex. force fitting using a machine). Certainly requires knowledge of product\'s design and intended functions of parts. Difficulty may cause assembly step to be incapable of performing until proper tools or knowledge are acquired. High level of complexity.

Figure 1. Table of difficulty ratings

Difficulty of Steps

Step #

Difficulty Level

Assembly Method

Evidence

Reasoning

1

3

Force fitting via robot/machine

Assembly requires great amount of force to accomplish.

Unable to disassemble or assemble part using simple tools.

2

1

Hand

Simple action, but components are oriented such that a robotic arm would have a difficult time getting to.

Arranged so that some hand dexterity is involved to hold the nut and screw wing on through part of foot.

3

3

Force fitting via robot/machine

Assembly requires great amount of force to accomplish.

Unable to disassemble or assemble part using simple tools.

4

2

Hand

Simple action, but requires human intuition.

Requires hand dexterity to feed wires with geometry at ends through a small slot.

5

2

Hand

Intermediate level of complexity

Requires human intuition to fit wires into the handle, different bends in wires make it difficult to program a robot able to perform this accurately.

6

1

Robot

Simple action

Can easily position product on assembly line and have a small robotic arm quickly insert and turn torx screws; holes are on outside of saw.

7

1

Robot

Simple action

Robot can easily pick up and insert parts into one another with large clearances.

8

1

Robot

Simple action

Robot can easily pick up armature-field assembly and insert it into the motor housing positioned on an assembly line.

9

1

Hand

Need fingers to push in springs and insert carbon brushes into slots while working around geometry.

Requires dexterity of human hand to perform.

10

1

Robot

Simple action

Product can easily be positioned on assembly line and have torx screws be quickly inserted and turned, holes on outside of saw.

11

1

Robot and hand

Simple action

Handles can easily be arranged in assembly line to be picked up by robotic arms and slid onto motor housing, but hands would be required to turn housing over and place screws in.

12

1

Robot

Simple action

Robot can quickly place housing on upper blade guard and turn screws.

13

1

Robot

Simple action

Small robot can quickly pick pinion shaft up and insert into center of saw on fast moving assembly line.

14

1

Robot

Simple action

Robot can easily pick up spring and insert into hole.

15

1

Robot and hand

Simple action, but requires human hand to fit in small confined space.

Human hand needed to manipulate spring and move lower blade guard so spring may be attached.

16

3

Hand

Complex action

Requires greater dexterity from hand to position lever and place c-ring on.

17

1

Robot

Simple action

Robot can easily place inner and outer washers on shaft and quickly place and turn blade nut into end.

18

1

Robot

Simple action

Product can travel down assembly line and robot can quickly apply and screw in lever.

19

1

Robot

Simple action

Screws may easily be inserted and turned into products on assembly line.

20

1

Robot

Simple action

Stopper is easily screwed into lower blade guard and can quickly be done on assembly line.

Figure 2. Difficulty rating of assembly steps

Assembly Guide

Step #

Description

Tool Required

Photograph

1

Insert rolling pin connecting foot to upper blade guard.

Hammer and T20 Torx Screwdriver

2

Screw wingnut into foot.

Hands

3

Insert bearing into lower blade guard.

Hammer

4

Feed cables into motor housing and attach to 120V field.

Hands

5

Insert power supply cables into handle.

Hands

6

Assemble handle halves using medium length torx screws.

T20 Torx Screwdriver

7

Insert 120V armature into 120V field.

Hands

8

Insert 120V armature and 120V field paired together into the motor housing. Attach 120V field to housing using medium length torx screws.

Place inner and outer washers onto pinion shaft and secure in place using blade nut.

Hands and 13 mm Torque Wrench.

18

Attach lower guard lift lever with a short torx screw.

Hands and T20 Torx Screwdriver

19

Insert small torx screws into upper blade guard.

T20 Torx Screwdriver

20

Attach rubber stopper to upper blade guard.

T20 Torx Screwdriver

Figure 3. Step by step assembly guide

\'\'\'The Reassembled Product\'\'\'

Figure 4. Reassembled circular saw

\'\'\'Total Assembly Time: 1 hour and 15 minutes\'\'\'

Reassembly and Dissection Process Comparison

After reassembling the product, it became apparent that the process was fairly similar to the dissection process. We followed the dissection process in reverse for some parts, but for others it was easier to assemble some components first before proceeding with the next step in the assembly. For example, we found that it was easier to assemble the motor and handle together before attaching it to the upper blade guard because it was a lot easier to try and attach two large solid pieces together than assembling the motor and housing, connecting it to the upper blade guard, and then trying to slide the handle on and attach it while the housing was attached to a heavy piece. In addition, we would have had to maneuver the wires connecting the trigger to the 120V field inside of the motor housing without actually being able to look inside and see them. It was also similar to the assembly in that we used the exact same tool set that was used during the product dissection in gate 2, but this was certainly expected because the same tools necessary to take apart components would be needed to put them back together. The most significant difference between the reassembly and the dissection was the difficulty level in removing the c-ring and placing it back on again at an inconvenient angle. During the dissection, this required the use of multiple pairs of hands and great force needed to be applied. During the reassembly, it was reattached in a matter of seconds. In all, some parts of the assembly were identical to the dissection, but there were certainly some parts that were far easier to perform during the reassembly.

Product Assembly Challenges

The only significant challenge that our group faced during the product reassembly was that we were missing another group member again. We overcame this by assigning one group member as the photographer, another as a documenter, and the remaining two members as reassemblers. This worked very well for us during the product dissection in the second gate and it proved to be very effective during this gate. Our group also came more prepared ahead of time by reviewing the steps of the dissection so that we could easily pinpoint which parts should be reassembled first. Another challenge we faced was that our group members had more restricted use of time in the lab during the assembly than when we had previously done the dissection. This was because some group members had other obligations that needed to be attended to later after the reassembly. This challenge was easily overcome by working quickly and splitting up tasks. In addition, photographs were taken and steps were written down very quickly so the assembly was executed as efficiently as possible. Given the time constraints, we worked very well together and reviewing the dissection process ahead of time definitely expedited the assembly process.

\'\'\'Mechanisms\'\'\'

The main mechanism of the circular saw is the motor. This circular saw, along with most other hand held power tools, utilizes what is known as a universal motor. Universal motors can operate on AC or DC power and they typically operate by using brushes (to transfer power) and are connected to a gear train. Another common motor is the induction motor. However these motors weigh much more, are larger and also usually more expensive. These qualities make an induction motor unfeasible, even though they are much quieter. The universal motor is comprised of four main parts. The first part is the “inner motor”, which is also known as the armature. The second is the “outer motor”, or electric/voltage field, and the last two parts are comprised of a pair of carbon brushes. More parts do exist, however they are not directly part of the motor.

The motor’s main purpose is to supply rotational kinetic energy to the circular saw’s blade. In order for this to happen, the electrical energy being supplied (AC) by the power source must be converted into mechanical energy. First the raw electrical energy is supplied to the motor through a pair of carbon brushes. The carbon brushes are needed to keep smooth contact with the motor while it rotates. Carbon brushes do degrade with time (making them non-ideal), however they are a cheap and an economical solution to an otherwise difficult problem. The electrical energy surges through the tightly wound copper coils in the inner and outer motors, creating an electromagnet. Due to the high voltage and the sheer number of coils present, a strong magnetic field is created in both. The magnetic fields begin to repel each other and a rotating motion quickly ensues. This rotational energy is taken from the motor and transferred via a drive shaft directly to the cutting blade. This direct connection creates a relatively efficient transfer of energy, and is also quite robust.

The following equations are of use in the design of small, universal motors.

Symbol

Quantity

Units

n

Speed

rad/s

n:s

Synch speed

rad/s

T:f

Full load torque, or torque at rated power

lbf-ft

W:k

Inertia

lbm*ft^2

t

Acceleration time

s

T:av

Average acceleration torque

lbf*ft^2

T

Torque

lbf*ft^2

W

Work

Btu

HP

Horsepower

hp

P

Number of poles for AC motor

N/A

RPM

Rotations per minute

N/A

% slip

Motor Slippage

N/A

freq

Frequency

s^-1

E

Efficiency

N/A

D

Distance

ft

V

Voltage

Volts

I

Current

Amperes

r

Radius

ft

I:ref

Inertia reflected back to motor

lbm*ft^2

I:load

Inertia of load

lbm*ft^2

RPM:load

Rotations per minute of load

N/A

RPM:motor

Rotations per minute of motor

N/A

F

Force

lbf

Figure 5. List of symbols used in equations relevant to small, universal motors

Quantity

Equation(s)

Work

W = F*D

Torque

T = F*D

T:f = (HP*5252)/RPM

Horsepower

HP = (T*RPM)/5250

HP = (V*I*E)/746

HP = (T*RPM)/5250

HP = (r*2*pi*RPM*F)/33000

Rotations per minute

RPM = 120*freq

High inertia load

t = (W:k^2*RPM)/(308*T:av)

T = (W:k^2*RPM)/(308*t)

I:ref = 2*I:load*(RPM:load/RPM:motor)

Synchronous speed

n:s = (120*freq)/P

Speed

n = (5250*HP)/T

Number of poles for AC motor

P = (120*freq)/n:s

Frequency

freq = (P*n:s)/120

Motor slip

% slip = 100*(n:s-n)/n:s

Figure 6. List of equations used in the design of small, universal motors

\'\'\'Design Revisions\'\'\'

Handle Extension

A design consideration that could affect serviceability of the circular saw is the inclusion of an extendable handle. The current model restricts the user to only being able to use the product within reach of their arms and an extendable handle could work around this issue by increasing the distance in which the user could operate the product at. This design revision would require a complete change in how the handle is attached to the housing of the saw in addition to how the main power cords would be connected from the trigger to the motor. The handle would be attached to the housing by a collapsible frame which may be drawn in or out and then locked in at the appropriate distance by the user. The distance at which the handle may be drawn would not be extremely long because that would greatly decrease the overall safety of the product’s usage because they would be offered less control. In addition, a handle that is too long would also result in greater stress on the materials in the handle by increasing the length of the moment arm that holds the saw. In order to accompany the change in the design of the handle, the manufacturing process of the saw will also need to change. The handle would still be manufactured using injection molding and the collapsible shaft that connects it to the housing would most likely be created using die casting. During assembly, the shaft would probably be connected in between the handle and the motor housing by use of joints. The joints would either be unrestricted in one plane of motion or be given the ability to be locked in place so that the user can choose how they prefer the product to behave as they maneuver it forwards and backwards.

The extendable handle increases the serviceability of the product by enabling the user to operate it at a greater range. This can be useful if the product is being used to cut a very large surface or if the product needs to be maneuvered in an area where the user doesn’t have direct access to. This can also be a beneficial for the user because it will prevent them from having to bend over while trying to cut the material over a long distance; instead they will be able to maintain their original posture and simply push the circular saw in the direction that they want to be cut which can help reduce the risk of accidents happening. The design revision also changes how the user can interact with their working environment by allowing them to access hard to reach places and still be able to cut material safely. This revision may be expensive on the levels of production and sales, but the user may find this feature particularly useful if they find themselves needing to easily cut large or out of reach objects.

Figure 7. Description of handle extension revision

Dust Collection

Another design consideration that would greatly increase the convenience of the circular saw for its user would be the inclusion of a way for sawdust and debris to be collected. During the use of the product, waste material is often blown off to the side of the working environment and is left to be cleaned up by the user after they are done. In order to work around this issue, a proposed design revision is to include a detachable vacuum hose that may be mounted to the end of upper blade guard farthest from the user. As the saw blade cuts through material, the dust and debris created that enter the end of the upper blade guard would be fed into the hose and be collected in an external container such as a reusable bag for later disposal. In order for the current model of the product to accept this design change, a hole would need to be created in the upper blade guard and a small extension would need to be made on the exterior of the upper blade guard where the hole is so that the hose would be attached to or inserted into it. This creation of the extension would result in revising the manufacturing process so that the mold used to create the guard would feature it during the casting process and the hole would need to be machined out using a tap and drill.

The dust vacuum would change the way that the user operates the product within their working environment by now requiring less consideration for clean up after the product’s usage. Prior the design revision, the user would need to decide if their environment would be suitable to generate waste in. The addition of the dust vacuum ensures that little no waste is generated, and so this consideration for the operating environment is reduced through this function. This revision will also increase the safety of the product because it will limit the amount of dust available to enter the motor housing. If dust accumulates in the motor housing, then it will reduce the effectiveness of the removal of thermal energy and can even start an electrical fire if high enough temperatures are reached. This issue is mitigated by the dust vacuum because it will transport a large majority of the waste material away from the product before it can have the chance to travel elsewhere. Economically, this addition will cost more to produce in the manufacturing process because more material will be used and the molds used for manufacturing the parts will be revised. In addition, the inclusion of an exterior power source and mechanism that drives the vacuum will cause the price of this extension to increase as well. In order to give the user more options, it would be recommended that this revision be made such that the user has the option to choose whether or not they want to use it. If not, a plug will also need to be produced in order to fill in the hole that is made for the vacuum tube. Though this revision will increase the price of the product, the average customer might find this function beneficial if it means that they have less material to clean up after using the circular saw.

Figure 8. Description of dust collection revision

Rolling Pins

One of the inherent problems with the circular saw is precision during cutting. This isn\'t so much of a problem concerning the mechanics of the circular saw, but rather it has mostly to do with human error during use of the product. Various mistakes and inconsistencies can be made while the user guides the circular saw down the cutting plane. These cutting mistakes can happen for a multitude of reasons, but the most common is due to a change in cutting angle. This happens very easily because the foot of the saw (the part that comes rests on the cutting plane) has a free range of motion (except up and down).

The design change that we have come up with would mitigate this issue. Our solution is to add two “rollers” to the front and back of the circular saw. The rollers would stretch the full width of the circular saw, unlike wheels which are only present near the corners. The rollers would increase stability for a moderate price increase over standard wheels utilizing axles. However, the rollers would also be more durable and increase performance as well. The rollers would be of a relatively small size and would not be more than an inch and a half diameter. The rollers also would be constructed out of plastic. This is the most viable option due to plastics durability and ease of manufacture, especially relative to its cost. The plastic rollers would be layered in a crisscrossing matrix of rubber to increase friction between the operating surface. This would help increase traction, thus further improving precision cutting abilities for a minimal increase in cost.

Many people would argue that this would increase the weight of the saw too much and that the rollers might get in the way. These two concerns will be addressed in this paragraph. Being made out of plastic, the rollers would actually add very little weight to the overall saw. As for dimensional and profile concerns, that has also been addressed. The foot of the saw will have wheel well like structures implemented in it, comparable to those found on cars. The main difference being that the wheel well would stretch across the entire width of the foot of the saw to accommodate the length of the roller. This coupled with the fact that the rollers would only be approximately one and a half inches anyway; the downward profile extrusion would thus only be three quarters of an inch. To compensate for this height change, the circular saw blade needs to be adjusted accordingly.

For an additional charge (in the range of 20-30 dollars) the saw could be given the feature of being self-propelled. A self-propelled circular saw at first might seem kind of unnecessary, however this is not true. By eliminating the need for the user to push the saw, yet another degree of human error is eliminated from the equation. This self-propelled feature, coupled with the rollers already in place will provide an unparalleled degree of precision and accuracy currently unavailable on the market. To make the saw self-propelled, an additional motor would need to be added. This motor would be geared to provide a very low rpm, but high degree of torque. The motor and motor housing would be located on the back right hand corner of the saw foot. This space is currently not in use, and placing a motor there would be unobtrusive to the user. The housing would also be relatively small, measuring approximately 4” x 3” x 3”. It would be “built in” to the foot to help decrease its profile further, along with being curved to fit the shape of the motor. It would also be made of metal to increase durability; however the metal would be thin to still make it cost effective. This motor would only turn the back roller, since providing power to both is unnecessary and inefficient. The main power cord would branch off from the circular saw housing and enter the secondary motor to provide electricity, and thus power.

These two changes would allow the circular saw to complete a much wider range of projects, and operate in different areas around the globe. Its higher degree of accuracy would allow it to almost match the precision of table saws, while still being very portable. These factors would make it a more viable option for different regions around the globe. In fact this could be a viable and cheaper alternative for many rural villages and undeveloped nations. The increase in precision is one of the main driving factors, but another big one is safety. The self-propelled function and rollers help to make the saw much safer to operate. Novice and professional users alike will have to worry much less while operating the saw, thus decreasing their likelihood of getting injured. The saw will also be less likely to slide side to side, or go off track, which can both be very dangerous in some circumstances.